sysconf: move in from cpp-cheat

arm baremetal: give more TTBR rationale

README.adoc

@@ -11887,6 +11887,25 @@ What is POSIX:
 * https://stackoverflow.com/questions/1780599/what-is-the-meaning-of-posix/31865755#31865755
 * https://unix.stackexchange.com/questions/11983/what-exactly-is-posix/220877#220877
 
+==== sysconf
+
+https://pubs.opengroup.org/onlinepubs/9699919799/functions/sysconf.html
+
+Examples:
+
+* link:userland/posix/sysconf.c[]
+* link:userland/linux/sysconf.c[] showcases Linux extensions to POSIX
+
+Get lots of info on the system configuration.
+
+The constants can also be viewed accessed on my Ubuntu 18.04 host with:
+
+....
+getconf -a
+....
+
+`getconf` is also specified by POSIX at: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/getconf.html but not the `-a` option which shows all configurations.
+
 == Userland assembly
 
 Programs under `userland/arch/<arch>/` are examples of userland assembly programming.

baremetal/arch/aarch64/dump_regs.c

@@ -22,10 +22,15 @@ int main(void) {
         uint64_t tcr_el1;
         __asm__ ("mrs %0, tcr_el1;" : "=r" (tcr_el1) : :);
         printf("TCR_EL1 0x%" PRIX64 "\n", tcr_el1);
+        printf("TCR_EL1.A1 0x%" PRIX64 "\n", (tcr_el1 >> 22) & 1);
 
         uint64_t ttbr0_el1;
         __asm__ ("mrs %0, ttbr0_el1;" : "=r" (ttbr0_el1) : :);
         printf("TTBR0_EL1 0x%" PRIX64 "\n", ttbr0_el1);
+
+        uint64_t ttbr1_el1;
+        __asm__ ("mrs %0, ttbr1_el1;" : "=r" (ttbr1_el1) : :);
+        printf("TTBR1_EL1 0x%" PRIX64 "\n", ttbr1_el1);
     }
 
     return 0;

userland/linux/sysconf.c

@@ -0,0 +1,13 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#sysconf */
+
+#define _XOPEN_SOURCE 700
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(void) {
+    /* Number of processors, not considering affinity:
+     * http://stackoverflow.com/questions/2693948/how-do-i-retrieve-the-number-of-processors-on-c-linux */
+    printf("_SC_NPROCESSORS_ONLN = %ld\n", sysconf(_SC_NPROCESSORS_ONLN));
+    return EXIT_SUCCESS;
+}

userland/posix/sysconf.c

@@ -0,0 +1,12 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#sysconf */
+
+#define _XOPEN_SOURCE 700
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(void) {
+    /* Maximum lengh of command line arguments + environment variables: */
+    printf("_SC_ARG_MAX (MiB) = %ld\n", sysconf(_SC_ARG_MAX) / (1 << 20));
+    return EXIT_SUCCESS;
+}